This invention relates generally to a method for producing hydrocarbons from hydrocarbon-containing hydrates and relates more particularly to an economical method for such production.
Methane and other hydrocarbons are known to react with liquid water (or brine) or ice to form solid hydrocarbon hydrates. These compounds are believed to exist in very large quantities in Arctic regions in gas-bearing sediments which lie between about 1000 and a few thousand feet below the earth surface. Therefore, these hydrates represent an enormous potential resource of hydrocarbons.
The Russians in particular have been keenly interested in recovering hydrocarbons from these vast reserves. As outlined by W. J. Cieslewicz, in "Some Technical Problems and Developments in Soviet Petroleum and Gas Production," The Mines Magazine, November, 1971, on page 15, several different methods of converting hydrate gas back into the gaseous state directly in the formation which were under study by the date of that paper included (1) artificially reducing the formation pressure, (2) increasing the formation temperature, and (3) pumping of catalysts into the formation. Regarding the first of these methods, that method (according to the paper) can be used only in deposits with high permeability; and a very large pressure reduction is necessary, particularly in gas deposits containing heavier hydrocarbon components. Regarding the second of these methods, according to that paper, the method can be accomplished by pumping water, steam, or hot gases into the deposit. And, regarding the third of these methods, of the many chemical substances tried by the date of that paper, methanol produced the best results in bringing about the release of free gas from the hydrate.
By the early 1970's, as disclosed in a series of five Russian papers, the Russians suggested injecting pressurized materials into a gas-producing well located below an in situ hydrate zone in the Messoyakha field in order to prevent the formation of or to free the area around the well-bore from hydrates which form and plug the well. The Messoyakha field is unique in that it is the only known field that has located within the same reservoir a (lower) free gas zone and an (upper) hydrate zone. They proposed injecting hydrate inhibitors (e.g., methanol or ethylene glycol or calcium chloride solutions or mixtures thereof) into the reservoir by means of hydraulic fracturing. In N. V. Cherskii et al., "Methods of Locating, Opening Up, and Exploiting Productive Horizons Containing Crystal Hydrates of Natural Gas (On the Example of the Messoyakhskoe Field)," Institute of Physical and Engineering Problems of the North, Academy of Sciences of the USSR, Siberian Division, Yakut Branch, Publishing House "Nauka," Novosibirsk, 1972, pp. 112-119, (on page 6 of the translation by Associated Technical Services, Inc.) the following was stated: "The most effective method of preventing hydrate formation in the bottom-hole zone and destroying previously formed hydrates is to inject inhibitor (preferably methanol) into the reservoir by means of hydraulic fracturing. Forced injection of methanol into the bottom-hold zone of the productive formation of two wells of the Messoyakhskoe field gave a six-fold increase in production." The largest documented single treatment volume recited in any of the five articles was the injection of 1374 gallons (in a series of treatments with a cumulative volume of 5284 gallons in 100 days) of 25% by weight CaCl.sub.2 solution into the free-gas zone of the Messoyakha field, as was described in S. A. Arshinov et al., "Hydrate-Free Production of Wells in Messoyakhsii Gas Field," Gazovoe Delo, No. 12, 1971, pp. 3-5 (at page 2 of the English translation by H. Altmann, June 1972). Such small volume treatments would be designed to dissolve hydrates occuring immediately adjacent to the well-bore and thus to perform near well-bore cleanup.
However, in a report issued by the Gas Research Institute covering a Gas Hydrate Workshop held in Denver, Colorado on February 1, 1979, at page 41 the statement was made that workers in some quarters felt that pressure reduction or the injection of alcohol or glycol probably would not prove viable for the recovery of natural gas from in situ hydrates for both technical and economic reasons; on the other hand, a more likely means was considered to be the injection of heat. Water (or brine) according to that report may be injected into the formation; and the hotter the water, the less will be required. Therefore, geothermal sources were a distinct possiblity.
The same article continued on page 43 that underground or in situ recovery of gas from hydrates should be environmentally acceptable, whereas mining--even if feasible--would be less so. For every molecule of methane in the hydrate form, 6, 7, or more molecules of water may occur; and this would present problems of dilution for solvent additives, as well as possible contamination of the waters remaining.
The article (on page 43) also considered the need for fracturing or rubblizing the hydrate-bearing formation, as an adjunct of production. The article pointed out that hydrate formations have little or no natural permeability and that techniques are needed to create flow channels in order to increase contact with the recovery agent. However, fracture by explosive or hydraulic methods was stated to be not regarded as a promising method of creating permeability. For explosive methods, (according to the article) the heat of explosion would liberate water which would be expected to refreeze unless temperatures could be sustained by the introduction of circulating water or gases. Hydraulic methods might be more satisfactory, according to the article, with the fracture sustained by sand particles. Thus, holding a crack open was still a goal.
And, as recently as June 16, 1981, in an article entitled, "Gas Imprisoned in Permafrost," Vol. 9, no. 115, in The Energy Daily, at page 4, the following conclusion regarding the effective stimulation of hydrate reservoirs was stated, "So far, no economic method has been devised for freeing the gas from its permafrost prison." Therefore, despite what has been known in the prior art, a need still exists for an economical method of producing hydrocarbons from hydrocarbon-containing hydrates.